Electrical coupling circuit



Nov. 22, 1932. w. A. MaCDoNALD ELECTRICAL COUPLING CIRCUIT Filed July19, 1950 l l IILIIINQIIIIIIIAII Patented Nov. 2v2, 1932 l UNITED STATESPATENT OFFICEv WILLIAM A. MACDONALD, OF LITTLE NECK, NEW YORK, ASSIGNORTO HAZELTINE CORPORATION, OF JERSEY CITY, NEW JERSEY, A. CORPORATION OFDELAWARE i ELECTRICAL COUPLING CIRCUIT Application filed July 19, 1930.Serial No. 469,040.

This invention pertains to high-frequency ent for the amplification toincrease with freelectrical coupling circuits adapted to transquency. Ithas been found, however, that this.

fer energy throughout a range in frequency effect goes 'too farin theopposite,directioi1, 'te I from an exciting circuit to a tunableabsorbi. e., the sensitivity"'actually decreassgwith l 5 ing circuit.The coupling circuit may be increase intuning frequency. lilyV utilizedfor connecting an antenna system to The coupling circuit of the presentinven'- the input of a radio receiver or for intertion proposes toovercome the above noted connecting successive stages of ahigh-freobjections by utilizing a two-winding transquency thermionicamplier or radio receiver. former for coupling a pair of, thermionic Aprimary object of the invention is to tubes in cascade relation, forexample, where- 6 0 provide a coupling circuit of the type specified intheprimary circuit is rendered inductivewhich in association with theelements conly reactive throughout the tuning range so nected thereby,will provide a sensitivity or that normally the sensitivity willincrease amplification characteristic varying autowith frequency. Topartially offset this matically with the frequency of tuning in aeffect, a coupling condenser is'connected be- 65 manner which is undercontrol of the detween the primary and secondary terminals signer. Morespecifically thelinvention is of like open-circuit polarity, as a resultof intendedto provide an approach toward unwhich there will be providedbetween primary form sensitivity throughout the tunable freand secondarycircuits a capacitive coupling quency range. A which opposes themagneticcoupling between 7o It is well known that the connection of anwindings throughout the tuning range. antenna system to a rvadO receiverOI' the It will be obvious with this arrangement interconnection ofsuccessive stages of a highthat; whereas Jehe magnetic coupling betweenfrequency thermionicamplier by means 0f circuits will remainsubstantially constant the usual high frequency transformers haV-throughout the frequency range, the vcapaci- 75 ing tunable secondaries,results in an overall tive coupling will increase steadily With thesensitivity characteristic which increases with tuning frequency, andwill Jhns oppose the frequency throughOnt the tllnlng Tange effect ofthe magnetic coupling to a greater This condition is undesirable sinceit renders und greater degree as the upper frequency an amplifying 0rradio IeCeVng SYStem mOst limit is approached. Thus, whereas the ca- 80Sensitive et the uPPel fefluency hmlt WheIIe pacitive coupling has verylittle effect upon it is 16B-St stable If the design is such es theamplification at the low frequency limit, t0 insure adequate stabilityet the PPPeI' fre" it operates strongly to reduce the sensitivityquencies, the device becomes inefficient at the otherwise present et theupper frequency lower frequenCeS 0f tllnlglimit and thereby provides anapproach to- 85 Attempts have been mede t0 @effect ths ward uniformsensitivity throughout the tun- COHOU by pfevdmg 11? shunt lth the ablerange. Itis thus possible to design the Primary transformer Yndmg eseielent ce receiver for eicient amplification at the low- Peety t0Tender the Plmefy en eult resonant er frequencies consistent with stableoperaat a frequency below the tuning range, S0 tion et the upperfrequencies. 90 thatv it is capacitively reactive and hence In Order toprovide a maximum opposing possesses an impedance characteristicdeen'eet between the capacitive and magnetic creasing with increase infrequency throughcouplings as the upper frequenoy linut .is out thetunable range. This decrease in priapproached, it is desirable toarrange. 1n

mary circuit impedance coupled with the shunt with the primarytransformer winding 95 automatic decrease in effective coupling beasufficient oepeeity to render the primary tween primary and secondarycircuits, due to circuit resonant at a frequency slightly .above theshunting' effect of the capacity, as the the tuning range. To this'endthe primary tuning frequency is increased, will operate coil may be sowound that the various capacito offset the natural tendency otherwisepresties naturally associated therewith, such as 100 the distributedcapacity between turns, the coupling capacity betweeA primary andsecondary circuits, and the capa "ity ot an antenna circuit or theanode-to-gi-ound capacity of a thermionic tube ccnnectcd thereto, willrender the primary' circuit resonant at the desired frequency.

Referring now to the drm/'ing5. Figure l shows in diagrammatic forniradio receiver employing the coupiing circuits ot the present inventionfor connecting the antenna circuit to the receiver input and forinterconnecting successive stages thereof.

Figure 2 shows a seri-es of graphs indicating the manner in which thesensitivity varies with the frequency ot tuning` for various couplingcircuits in accordance with the present invention.

Fig. 3 shows a coupling circuit in accordance with the present inventioninterposed between thermionic tubes ci the three electrode type whereinthe ceuplin'g transformer is provided with a neutralizing wind.- ing andassociated condenser tor neutralizing the grid-cathode tube capacity.

In the circuit of Figure i, an antenna l and ground 2 are connected toopposite terminals of a variable resistance 3. the voltage drop acrosswhich is impressed through a coupling circuit A1 upon 'the input sectioncomprising grid and cathode of a thermionic.

tube V1 of the screen grid type operating as a stage of radio-frequencyamplification. In the case of the antenna coupiine; system, the primarycircuit being resonant above the tunable frequency range will becapacitively reactive throughout the range. The output sec'- tioncomprising anode and cathode of tube V1 is connected through a couplingnetwork A2 to the input section of a. second tube V2 also operating as astage of radio-frequency amplitication. The output of tube V2 isconnected through the usual detector and audiofrequency amplifier D to aloud speaker L. The proper operating potentials for tubes V1 and V2 aresupplied over conductors 4 while condensers f serve to bv-pass theradiofrequency currents around the voltage supply circuit. The iilamentsof the tubes may be heated by any of the well-known methods (not shown).

Each coupling` circuit A includes a transformer T having separatemagneticallyT coupled primary and secondary windings L1 and L2. In shuntwith the secondary winding is a variable condenser C2 which serves totune the circuit over a range in frequency, specifically the broadcastband. The condensers Co are mechanically coupled for simultaneousadjustment by means of the unicontrol device U.

The primary and secondary windings are in each instance arranged to havetheir ungrounded terminals of like open-circuit polarity so that byconnection of a coupling capacity C therebetween, a capacitive couplingwill exist between primary and secondary circuits which opposes theeffect of the magnetic coupling therebetween for the condition that theprimary circuit is inductively reactive throughout the tunable'frequency range, (except when the coupling system is a part of. theantenna circuit as previously explained) the coupling capacity C beingrsufiiciently small that the current flowing' therethrough is in phaseopposition to vthe current of the primary circuit.

As the tuning condenser C2 is adj usted for higher frequencies, themagnetic coupling M remains substantially constant, whereas the opposingcapacitive coupling1 due to C steadily increases. The combination ofthese two factors causes the resultant etiective coupling betweenprimary and secondary circuits automatically to decrease with increasein tuning frequency and thereby to oiiset in part the normal tendencyfor the sensitivity to increase with frequency.,

In order to render the effect of the opposed capacitive and magneticcouplings more pronounced as the upper requencylimit is approached, theeffective inductance of the primary winding L1 may be tuned by means ofthe capacities associated therewith to a frequency slightly above Athcupper Jtrequency limit of tuning. The effective inductance ot theprimary winding is the magnitude thereof obtained with the secondarywinding shortcircuited and is, in tact, equal to (l-KZ) Ll, where L1 isthe inductance ot the primary with the secondary on open circuit, and Kis the coefficient of magnetic coupling between circuits.

In the case of the couplingr circuit A1 the capacities operating torender the primary circuit resonant at a frequency above the tuningrange are (l) the capacity to ground of the antenna system proper, (2)the distributed capacity between turns of the primary winding L1, and(3) the capacity of the coupling condenser C to ground through thetuning and by-passing condensers C2 and f, respectively, which latterare so large in comparison with C that their effects may be neglected,and (4) any stray capacitive effects existing between the terminals ofthe primary Windinv.

In Figure 1 all of the capacities existing across the primary winding L1with the eX- ception of the coupling capacity C are indicated by thelumped capacity C1. The total capacity thus associated with the primarywinding should be suiiicient to tune the primary circuit slightly abovethe tuning range, and if the capacities naturally associated with theprimary winding are not suiiicient to this end, a supplemental condensermay, of course, be connected thereacross.

In the interstage coupling circuitlA2 the primary circuit is tuned toresonance above turns and the anode-to-ground capacity of e tube V1connected thereto, theseff-latter two capacities together with any.stray capacitive'eifects being indicated by the lumped capacity C1. l

The coupling capacities CI physical condenser units or the primaryand'secondary transformer windingslmay be so wound and so positionedthatihefnatural capacities existing between win ings--willproduce thedesired results.

Figure 2 shows a set of curves'obtained eX- perimentally depicting themanner'inwhichv` ythe coupling capacity Cvcontolsthe.ampli- -cation perstage as the -frequen o adjusted throughoutthe range The f lcurve C=0representsthe conditi l coupling capacity and shows clearly tlfxeman-`ner in which theesensitivity/nrrnally-'inwai@ and 1/ in the order named,'showthe' esults creases with frequency. ThecurvsC obtained by utilizingcoupling capa ities 'of successively increasing magnitude: From thesecurves it is apparent thatfth "coupling capacity has substantiallynoeifectfonfthe sensitivity at the low-frequenc'yfend.gf'ffthe scale,but reduces -it below its normalfvalue by a steadily increasing amountas the. fre. quency of tuning is adjusted toward the upper frequencylimit. Within limits it is also apparent that the greater the couplingcapacity, the more nearly a condition of uniform sensitivity isapproached.

Other factors remaining constant, the coupling capacity cannot beincreased in magnitude indefinitely with attendent improveyments asregards uniform amplification. As

pointed out above, the capacity C is one of the factors determining theresonance frequency of the primarycircuit which must always bemaintained of the order of the upper frequency limit, since otherwisethe ampliication will rise to a maximum value within the tuning range`and then drop off-sharply `thermionic tubes V1 and V2 of thethreeelement type are coupled in cascade relation by means of a couplingcircuit A3 the ampli- Vfication'cr gaincharacteristics of which aresimilar to that of circuit A1 and A2. In order,

however, to neutralize the grid-anode capacity of tube V1 transformer T3in addition to the primary and secondary windings L1 and L2, is providedwith a neutralizingwinding LN magnetically coupled thereto. The windingLN is connected from the low potential terminal of winding L1 through aneutralizfr ing capacity `CN to the grid electrode of tube f tuning' V."iT-liel neutralization isy accomplished in the'A well-known mannerdescribed in Patent No. 1,533,858 to L. A. Haifeltine.

What isfjclaimed is:

1. A high-frequency electrical coupling system. tunable over a range infrequency comprising separate primary and secondary circuits, saidcircuits being coupled magnetimeans of primary and secondary idling'throughout "said frendsaid primaryf transformer nv the operation ofsaid sysimary lcircuit resonant at a he hihest frequency of said ...Afl- 1 quencyelectrical coupling ,t'emtunabiep' r aran'ge in frequencycomprisingjsparate"primary and secondary cireuits,.`sa circiijitsbeingcoupled magnetically byvm'eiinsoftransformer windings individualtheretgfand coupled capacitively by Ymeans offa' condenser connecting aterminal of eacl1,winding,'a conductive path extending between two"remaining terminals of said windings, said capacitive coupling beingarranged to oppose said magnetic coupling throughout said tunable range,and said primary winding in the operation of said circuit being arrangedto have capacity associated therewith rendering said primary circuitresonant at a frequency above the highest frequency of said tunablerange.

3. A high-frequency electrical coupling system tunable over a range infrequency comprising separate primary and secondary circuits, saidcircuits being coupled magnetically by means of primary and secondarytransformer windings individual thereto, respectively. and coupledcapacitively by means of a condenser connected between winding terminalsof like open-circuit polarity. said primaryA winding in operation ofsaid system having capacity associated therewith renderquency slightlyabove the highest frequency of said tunable range, whereby the voltageamplification of said system remains substantially uniform as the tuningfrequency is adjusted throughout said range.

4. A hi h-frequency electrical coupling system for interconnectingelements of a er windings individual thereto, re-` `coupled capacitivelyby means' ectin'gri said circuits, said' A being arranged tooppose.

pacity associa-ted therewith.

lng said primary circuit resonant at'a frese'v radio receiving systemcomprising a transformer having separate magnetically coupled primaryand secondary windings, a variable condenser bridging said secondarywinding for tuning the system over a range in frequency. a capacity7connecting terminals of said windings of like open-circuit polarity, anda conductive path extending between two remaining terminals\thereof,said primary Winding in the operation oit said device havinjr capacityassociated therewith render ing the primary circuit resonant a frequencyslightly above 'the highest frequency of the tuning range whereby saidcapacitive coupling opposes #said magnetic coupling between 'windingsand the aniplircation produced by said system approaches uniformitythroughout said tunable range.

5. In combinatioin an electrical coupling circuit interconnecting anantenna circuit with the input section et a therinonic tube in a ra dioreceiver, said coupling system comprising separate magnetically coupledprimary and secondary transformer windings connected to said antennaandsaid tube respectively, a variable condenser bridging said secondaryWinding for tuning said circuits over a range in frequency, a.capacitive coupling extending between said windings arranged to opposesaid magnetic coupling throughout said tunable range, and capacityincluding the capacity of said antenna and oiz said capacitive couplingassociated with said primary circuit rendering the prirnary circuitresonant at a frequency above the highest frequency of said frequencyrange.

6. In conibinatioin radio-frequency coupling circuit interposed. betweenelements of a therniionic tube radio-frequency amplifier, said circuitcomprising separate 1nagnetically coupled primary and secondarytransformer windings connected to said eienients respectively, a.variable condenser associated with said secondary for tuning saidcircuit over range in frequency7 a capacitive couu pling extendingbetween said windings arranged to oppose .said magnetic couplingthroughout said requency range7 and capacity associated with saidprimary Winding rendering the primary circuit resonant at a frequency ofthe order of the upper limit of said tunable range.

7. In combination, a pair of thermionic tubes, each including cathodeand grid constituting an input section, and an anode and said cathodeconstituting an output section, a high-frequency electrical couplingcircuit connecting the output section of one said tube to the inputsection of the other, said circuit comprising separate magneticallycoupled primary and secondary windings connected to said sections,respectively, a Yvariable condenser bridging said secondary winding fortuning said circuit over a range in frequency, a. capacitive couplingextending between said windings arranged to 0ppose said magneticcoupling throughout said frequency range, the eective inductance of theprimary Winding being such, in conjunction with the capacity associatedtherewith which includes the anode-to-cathode capacity of the tubeconnected thereto and said coupling capacity, as to render the primarycircuit resonant at ire ency of the order of will@ f reef my signature.a. linoJNl-LQ 'ilO

